Vacuum drying system with cryopumping of solvent recovery feature

Abstract

In the drying of solvents-containing material in a vacuum concentrator and which involves condensation of evolved solvents in a cold trap, simplified and inexpensive flow blocking devices are provided intervening an outlet of the cold trap and an inlet to the vacuum pump with which the concentrator is evacuated. The flow blocking practiced allows cryopumping effect to ensue between the concentrator and cold trap such being the agency by which solvent vapor outflows from the concentrator and is recovered by condensation thereof in the cold trap. Near complete solvent recovery can be achieved and carryover of solvent to the vacuum pump is reduced to insignificant measure. In one form, flow blocking is attained by use of a simple pressure differential valve. This valve will stay closed as long as absolute pressure differential acting on the valve is a desired value, such as one corresponding to difference of condition of vacuum at pump inlet and at the upstream end of the valve, such differential being one at which cryopumping conditions exist. If the differential is greater, the valve will open to reconnect system flow to the vacuum pump which continues until cryopumping condition is again achieved when the valve will again close. Flow blocking also readily can be provided with a solenoid operated valve, the solenoid being controlled by a pressure differential switch sensing condition of vacuum at the pump inlet and at a location upstream in the system. When the differential of vacuum conditions sensed is as desired, the switch is closed maintaining a signal to the valve keeping it closed allowing cryopumping of solvent to occur. When the sensed differential is greater than desired, the switch opens, the signal terminates and the valve opens to connect the system to the vacuum pump.

What is claimed is: 
     
       1. Apparatus for removing solvents from a solvents-containing material in the course of a drying operation, comprising a chamber in which material to be dried is received,   a vacuum pump,   a conduit connecting the chamber with the vacuum pump, the vacuum pump being operable to produce a condition of vacuum at an inlet to the pump and predetermined conditions of vacuum at a number of locations in said conduit upstream of the pump inlet which predetermined conditions of vacuum successively have values increasingly less than the pump inlet vacuum condition the more remote the location from the pump inlet so that successive expected absolute pressure differentials exist between the pump inlet and successive ones of the locations,   a cold trap in the conduit downstream of the chamber into which solvent caused to evolve from the material under the influence of vacuum acting thereon can flow and be condensed, the cold trap being upstream of the pump inlet,   a flow control element located in the conduit intervening the cold trap and the pump inlet and positionable to have flow passing and flow blocking orientations, and   flow control element orientation means operable responsive to presence of a greater differential of absolute pressure between the pump inlet and a given one of said locations than an expected differential to produce actuation of the flow control element to flow passing orientation, said means being operable responsive to presence of an expected or less than expected differential of absolute pressure between the pump inlet and said given one location to produce actuation of the flow control element to blocking orientation.   
     
     
       2. The apparatus of claim 1 in which the flow control element and the flow control element orientation means are embodied as a differential pressure valve. 
     
     
       3. The apparatus of claim 2 in which the differential pressure valve is a spring-loaded check valve. 
     
     
       4. The apparatus of claim 1 in which the flow control element includes a housing having an inlet end and an opposite outlet end, the housing inlet end being in communication with one of said conduit locations upstream of the pump inlet and the housing outlet end being in communication with the inlet to the pump, and a valve element movably mounted in said housing for movement between flow blocking and flow passing orientations, the orientation means including a spring member engageable with the valve element for urging it into flow blocking orientation, said valve element having oppositely located surfaces exposed to vacuum condition presence at the housing opposite ends whereby differential of absolute pressure acts on said valve element tending to move it, the spring member having urging characteristics sufficient to urge the valve element into flow blocking orientation whenever the said expected or less than expected differential of absolute pressure is present but being insufficient to prevent movement of the valve element to flow passing orientation when greater differential of absolute pressure between that at the pump inlet and that at a location upstream of the pump inlet is present. 
     
     
       5. The apparatus of claim 4 in which the housing includes a seat containing a port, the valve element in blocking orientation being urged against said seat in port covering disposition by the spring member. 
     
     
       6. The apparatus of claim 4 in which the urging characteristics of the spring member are such as to hold the flow element in flow blocking orientation whenever the expected differential of absolute pressure condition is more than about 50 Torr. 
     
     
       7. The apparatus of claim 4 in which the urging characteristics of the spring member are such as to hold the flow element in flow blocking orientation whenever the expected differential of absolute pressure condition is more than about 10 Torr. 
     
     
       8. The apparatus of claim 1 in which the flow control element is a power operated device, the orientation means including a pressure differential sensing switch actuatable to a switch closed position whenever expected or less than expected differential of absolute pressure is sensed by the switch, said sensing switch in a closed position thereof being operable to signal application of power to the flow control element. 
     
     
       9. The apparatus of claim 8 in which the sensing switch has a pair of ports at opposite sides of a movable sensing member, one such port being communicated with the pump inlet, the other port being communicated with the said given one location. 
     
     
       10. The apparatus of claim 9 in which the said given one location is upstream of the cold trap. 
     
     
       11. The apparatus of claim 8 in which the flow control element is a solenoid operated valve. 
     
     
       12. Apparatus for removing solvents from a solvents-containing material in the course of a drying operation, comprising a chamber in which the material to be dried is received,   a vacuum pump,   a conduit connecting the chamber with the vacuum pump, the vacuum pump being operable to produce a condition of vacuum at an inlet to the pump and predetermined conditions of vacuum upstream of the pump inlet which have values which are less than said pump inlet vacuum condition, and   a cold trap in the conduit intervening the chamber and the pump inlet, solvent caused to evolve from the material under the influence of vacuum acting thereon flowing into and being condensed in the cold trap,   a differential pressure valve in the conduit intervening the pump inlet and an outlet of the cold trap, the valve embodying a spring urging a closure element in a flow checking condition whenever an absolute pressure differential between that at the pump inlet and at a conduit location immediately upstream of the valve is at or less than a differential between a vacuum condition at the pump inlet, and a predetermined condition of vacuum at said immediately upstream location.   
     
     
       13. The apparatus of claim 12 in which the spring is effective to maintain the valve in checking condition whenever the differential of vacuum condition between the pump inlet and the upstream location is 50 Torr or less. 
     
     
       14. The apparatus of claim 12 in which the differential pressure valve is a spring-loaded check valve.